ETYM Old Eng. sodian, sodein, Old Fren. sodain, sudain, French soudain, Latin subitaneus, from subitus sudden, that has come unexpectedly, p. p. of subire to come on, to steal upon; sub under, secretly + ire to go. Related to Issue, Subitaneous.
Happening without warning or in a short space of time.

An unexpected occurrence; emergency